The present invention relates to a method for preparing sputter target/backing plate assemblies, and to the target/backing plate assemblies prepared therefrom. More particularly, the invention pertains to methods for diffusion bonding copper sputter targets to associated backing plates, preferably composed of aluminum, aluminum alloy, aluminum matrix composite material, copper, or copper alloys, using nickel-alloy interlayers and to assemblies produced thereby.
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits to form the thin layer or film on the receiving surface of the substrate.
In so-called magnetron sputtering, a magnet is positioned behind the cathode assembly to generate an arch-shaped magnetic field. The arch-shaped field, formed in a closed loop configuration over the sputtering surface of the sputter target, serves to trap electrons in annular regions adjacent the sputtering surface. The increased concentrations of electrons in the annular regions adjacent the sputtering surface promote the ionization of the inert gas in those regions and increase the frequency with which the gas ions strike the sputtering surface beneath those regions.
The sputter target is heated during the sputtering process by the thermal energy of the bombarding gas ions. This heat is dissipated by heat exchange with a cooling fluid typically circulated beneath or around a backing plate which is bonded to the sputter target along an interface opposite the sputtering surface.
With respect to sputter target assemblies used during sputtering processes, more specifically to copper target assemblies, these assemblies historically have been made by bonding a high-purity Cu plate to a lightweight and highly heat conductive backing plate, such as Al, aluminum alloy, or aluminum matrix composite materials. Additionally, Cu targets may be bonded to a Cu, or Cu alloy, backing plate in forming target assemblies. Two methods used in bonding Cu targets are solder bonding and diffusion bonding.
Solder bonds typically tend to produce relatively low bond strengths. More specifically, solder bonding between a Cu target and an Al backing plate only produces up to 4,000 psi (28 Mpa) bond strength with bond strength decreasing as the temperature of the target assembly increases during sputtering. In addition, typical solder bonding has low melting point and high vapor pressure elements that are a potential source for contamination in a sputtering chamber during sputter deposition. Therefore, diffusion bonding is preferred.
However, the bond between a Cu target and backing plates formed of various metallic materials may produce very brittle intermetallic compounds during the bonding process resulting in a weak bond. Specifically, the bond between Cu and Al produced by diffusion bonding is extremely weak due to the fact that Cu and Al form several very brittle intermetallic compounds during the bonding process. These brittle interphases tend to reduce the mechanical load necessary to initiate failure during tensile testing, for example, copper and aluminum are capable of forming a low temperature intermetallic phase incapable of withstanding a tensile stress greater than approximately 2 ksi (1.4xc3x97107 N/m2).
To eliminate this intermetallic interphase and improve bond strength, it is necessary to use an interlayer, such as Ni or Ti, between the target and backing plate.
Nickel has been used previously as an interlayer to eliminate the intermetallic interphase. The use of Ni interlayers eliminates the intermetallic phase between target and backing plate thereby significantly improving bond strengths especially between copper sputter targets and aluminum backing plates.
Notably, Ni interlayers may be prepared by sputter deposition. However, sputter deposition of a Ni interlayer is difficult because Ni is a highly ferromagnetic material. Accordingly, usage of pure Ni results in low magnetic flux intensity in front of the target surface because a considerable portion of magnetic flux from system magnets is shunted by the target. Magnetic targets cannot be efficiently sputtered, nor plasmas be ignited, if there is not sufficient magnetic flux intensity in front of the targets.
A Ni-alloy interlayer can be used to achieve higher magnetic flux intensity and improved magnetic flux distribution over pure Ni interlayers. Specifically, alloying Ni with V, Ti, Cr, and Si can reduce the magnetic permeability of Ni. Using such Ni-alloy interlayers significantly increases the deposition rate and allows the use of thicker targets in production. Thicker targets require less frequent target replacement resulting in decreased tool down time and increased productivity. Also, layer uniformity is improved which increases bonding quality and strength. Lastly, a fine grain and preferred crystallographic texture in the bonded Cu target is retained.
Accordingly, there remains a need in the art for a sputter target assembly comprising a high purity copper sputter target, a metallic backing plate, preferably composed of aluminum, aluminum alloy, aluminum matrix composite material, copper, or copper alloys, and a Ni-alloy interlayer having a high bond strength and no intermetallic phase between target and backing plate, and a method of making same.
The present invention provides a sputter target/backing plate assembly comprising a sputter target composed of copper and alloys thereof, most preferably high-purity copper, a backing plate composed of a metallic material, preferably aluminum, aluminum alloy, aluminum matrix composite materials, copper, or copper alloys, and an interlayer between the target and backing plate wherein the interlayer, target, and backing plate are diffusion bonded together. The interlayer is composed of a Ni-alloy, preferably Nixe2x80x94V, Nixe2x80x94Ti, Nixe2x80x94Cr, and Nixe2x80x94Si.
With respect to the target assembly, higher magnetic flux intensity and improved magnetic flux distribution can be achieved by using a Ni-alloy target. Alloying Ni with V, Ti, Cr, and Si can reduce the magnetic permeability of Ni. Also, using such Ni-alloy interlayers significantly increases the deposition rate and allows the use of thicker targets in production. Thicker targets require less frequent target replacement resulting in decreased tool down time and increased productivity. Lastly, layer uniformity is improved which increases bonding quality and strength.
The method of making the sputter target/backing plate assembly of the present invention includes depositing, preferably by means of electroplating, sputtering, or plasma spraying, the interlayer on one of the mating surfaces of the target or backing plate. The mating surface of either the sputter target or the backing plate preferably is roughened to form a plurality of salient portions projecting therefrom. The mating surfaces must be carefully cleaned prior to deposition of the interlayer. After deposition, the sputter target and backing plate are pressed together along the mating surfaces. The sputter target and the backing plate are held in contact at a temperature just below the melting points of the sputter target and backing plate materials to promote diffusion bonding. In a preferred method of making, the target, interlayer, and backing plate are HIPped together in a HIP can.
In an especially preferred form of the invention, the mating surface on the harder of the sputter target or the backing plate is roughened by machining a series of concentric grooves therein. The grooved mating surface is covered by a nickel-alloy layer having a consistent thickness, preferably of approximately 1 to 100 microns. The sputter target and the backing plate are then joined along the mating surfaces using hot isostatic pressing. The process results in a fully joined assembly wherein the sputter target material is bonded in situ to the backing plate material through the medium of the nickel-alloy interlayer.
Preferably, the nickel-alloy interlayer is of sufficient thickness that the sputter target and the backing plate form substantially separate metallurgical bonds therewith; that is, the assembly forms two relatively strong intermetallic phases, including a first intermetallic phase between the nickel-alloy and the backing plate and a second intermetallic phase between the nickel-alloy and the copper of the sputter target, rather than a single, relatively brittle intermetallic phase between the sputter target and backing plate materials themselves.
Typically, the high purity metal or metal alloy of the sputter target will be harder than the metallic material of the backing plate. When the grooves and the nickel-alloy interlayer are formed on a mating surface of a sputter target formed of a metallic material harder than the metallic material from which the backing plate is composed, the jagged salient portions or ridges defined between the grooves serve to puncture the native oxide on the aluminum backing plate. The sides of the grooves provide additional surface area across which the metallic material from the backing plate diffuses to form a thick, yet strong, intermetallic phase.
The disruption of the mating surfaces caused by the pressing of the roughened surface against a substantially planar mating surface is believed to provide additional mechanical advantages. It is believed that the non-planar interface between the sputter target and backing plate portion of the completed assembly increases the resistance of the bond to shear failure. Furthermore, it is believed that the salient portions defined by the grooves fold over during the pressure consolidation of the assembly to form a locking type mechanism.
Whether on the mating surface of the sputter target or of the backing plate, the roughening provides the added advantage of facilitating the deposit of the nickel-alloy interlayer through the removal or disruption of any oxide layer formed on that surface.